Simple frequency converter - a TBA 120 application

During the last few years the TBA 120 has become one of the most frequently used ICs in RF techniques. Although originally meant as IF amplifier/FM demodulator, the TBA 120 can be used for a wide range of applications. This converter circuit is just one example. The initial requirements for a converter are a mixing stage and an oscillator. The multiplier in the IC suits the needs of a mixing stage perfectly well. The oscillator can be realised by a selective (positive coupling) feedback of the amplifier section of the TBA 120 by means of the resonance circuit L1/C1. The oscillator will operate at a frequency of 46 MHz with the values indicated in the circuit diagram. Consequently, we are dealing with a circuit that converts an input signal of 35.3 MHz into 10.7 MHz (46 — 35.3 = 10.7 MHz). This can be used to convert the IF signal of a TV tuner into the intermediate frequency of an FM receiver. Obviously the circuit can also be applied for other frequencies, by modifying the osci...